A complete system for adaptive control of electrical discharge machining has long been sought after. Different forms of partial adaptive control have been offered in commercial equipment in which, for example, one of the parameters of machining, such as machining pulse on-time or off-time, may be controlled by a feed-back signal from the gap. One example of a circuit in which on-off time control is exercised in response to gap voltage parameters is disclosed and described in my U.S. Pat. No. 3,705,969 issued on Dec. 12, 1972 for "Cut-Off Protection System for Electrical Discharge Machining Power Supply." Others incorporated systems in which, for example, peak current was limited in a system in which according to the preset frequency and on-time there were switched in one or more series resistors which operate to control the peak current magnitude applied during the particular machining operation. One example of a system of this type is shown in U.S. Pat. No. 3,737,615 issued on June 5, 1973 for "Current and Frequency Control system for Electrical Discharge Machining Apparatus."
A precision operating adaptive control system necessarily takes a number of control data inputs, whether these be from a manual input operated by the machine operator from a keyboard, thumbwheel switch, or from an operator encoded punch card, magnetic tape, or any other information carrier and reader known and now commercially in use. It is important that these inputs are of at least two types, including one for selecting surface finish and another for selecting wear ratio. Both of these inputs are related to the particular pulse on-time which is desirable for any selected combinations of material in the electrode and workpiece setup. The inputs may be entered manually by the operator into a control input device as the result of his own experience in earlier electrical discharge machining operations which he himself has run or, alternately, may be the result of a relatively great number of prior machining operations on which empirical data has been accumulated in a memory file relative to the surface finish, wear ratio and corresponding on-times most appropriate for a large number of different workpiece and electrode combination.
In the memory file controlled system of the present invention, this data is contained in a basic operating program and is available on demand to provide through a data select signal input for the programmable computer, which in turn will then select and control the proper parameters for operation of the digital type multivibrator, which in turn controls the on-off time or the off-time of the electronic output switch or switches in the system.
It will be understood in the specification that when I refer to "electronic switch" I mean any electronic control device having three or more electrodes comprising at least two principal or power conducting electrodes acting to control current flow in the power circuit, the conductivity between the principal electrodes being controlled by a control electrode within the switch whereby the conductivity of the power circuit is controlled statically or electrically without movement of mechanical elements within the switch. Included within the definition are transistors in which turn-on is accomplished by a control voltage applied to the transistor control electrode and in which turn-off is accomplished automatically in response to removal of that control voltage. Also included in the definition are devices of the gate type in which turn-on is accomplished by a control voltage applied to the control electrode, which control voltage may be then removed and in which turn-off is accomplished by application of a subsequent control voltage to the control electrode. An additional class of electronic switches, called "electronic trigger devices," falls within this definition and includes ignitrons, thryatrons, semi-conductor controlled rectifiers and the like. By "electronic trigger device" I mean any electronic switch of the type which is triggered "on" at its control electrode by a pulse and is turned "off" by a reverse voltage applied for a sufficient time across its principal electrodes.
It will be understood that the present system will operate effectively only when there is included in the EDM power supply a pulse generator of the digitally controlled type, preferably including a counter and a digital control system such as the type shown and described in my U.S. Pat. No. 3,809,847 for "Method and Apparatus for Electrical Discharge Machining" issued May 7, 1974. In that patent, there is shown a system whereby the desired on-time and off-time can be separately dialed by the operator on decimal-type thumbwheel switches, which switches then furnish a continuing visual indication of the on-off times of the machining operation then being carried on. Such a digital-type pulse generator is particularly suitable for incorporation with the system of the present invention.